1. Field of the Invention
The present invention relates to an optical ingredient-measuring apparatus for optically measuring a concentration of a substance having the optical rotation (optical activity) such as glucose, more particularly, to an optical ingredient-measuring apparatus with the application of a Sagnac interferometric system (interferometer).
2. Related Art
It has been known that the optical rotation (optical activity) of a optically rotative substance such as glucose depends upon the concentration of the substance. Therefore, the sensing of the concentration of such a substance has been carried out by measuring the optical rotation of the substance and calculating the concentration based on the measured optical rotation.
As the method for measuring the optical rotation (or birefringence) of the substance, various kinds of conventional methods such as Senarmont method, orthogonal polarized wave differential method, Muller matrix calculation method, orthogonal polarized wave heterodyne method, modulation phase shift method, wavelength sweep polarization degree measuring method, polarization degree (minimum value) measuring method have been known.
However, since these methods relate to methods for measuring a rotation angle of a plane of polarization (polarization plane) by the optical rotation in a direct manner or with the use of a polarization degree, there are disadvantages in that an angular resolution thereof is low, and that a measurement error is large. Further, there are additional problems in that the measuring apparatus is large-scaled, the elements used for measuring are expensive, the time is needed for adjusting an optical axis, and therefore the measuring apparatus is expensive.
Thus, Japanese Patent Laid-Open No. 2005-274380 (JP-A 2005-274380) proposed an optical ingredient-measuring apparatus with the application of the Sagnac interferometric system that has been used in an optical fiber gyro, etc.